1. Field of the Invention
The present invention relates to motor-driven antennas for automobiles.
2. Prior Art
In conventional motorized antennas for automobiles, a rack rope, attached to the lower end of a rod antenna, is moved in a vertical direction by gears which are driven by a motor. In addition, a clutch is used between the motor for driving the antenna and the rack rope in order to minimize damage to other components such as the rack rope when the rod antenna is locked.
In prior art antennas, when the power of the automobile radio is turned on, the motor for the antenna starts to rotate, and the rod antenna is extended gradually. When the antenna reaches its maximum length, it is mechanically locked. When the rod antenna is thus locked, the current in the motor increases. Based upon such increased motor current level, the locking of the antenna is detected. At this moment, the clutch is disengaged, thereby interrupting the electric current to the motor.
In the above-described conventional apparatus, locking of the antenna is detected based on the electric current flowing to the motor. However, if a malfunction occurs in the circuit which detects the motor current, the clutch continues to operate while being disengaged emitting a clacking noise. Furthermore, the motor wastes power during the period when the clutch is disengaged.